Why would she just ignore me?

A few years ago my girlfriend broke up with me and never told me why. As I completed my last year of high school, she talked me around school and at my work. I was young and stupid and didn't handle the situation right. I reconnected with a mutual friend and she told me what actually happened. My ex thought I was lying about a group of girls at school and I was doing something with them. It was a lie.

So I messaged her on Instagram and after two weeks, I saw she hadn't 'seen' it and I sent her a message in Twitter saying I had sent her a message on Instagram and I wanted her to read it. After a week, she had posted a lot and I knew she just ignored it. I deleted the message on Twitter and just let it be...

This is what I sent her:

Hey, sorry to bug you, but I sent a message to you on Facebook trying to apologize for what happened when we were in high school. I'm sorry I lied to you, and I'm sorry for all the stupid things I did. I can't say I was young and dumb, because I knew how dumb I was. I should have never asked you out when I didn't have a phone and had the restrictions from my parents. I handled everything between us in high school wrong, and I deeply apologize. I don't mean to pester or annoy you, like I Have In the past, but I just felt like I needed to apologize, I know it may not mean much. But I'm sorry.

Why would she stalk me, but when I tried to at least apologise, she couldn't bother to read it?